This invention relates to a control device for an AC motor using an inverter circuit.
Control devices for AC motors may be of many different types, depending on the type of the AC motor to be controlled.
This invention is concerned with a device for controlling induction motors and brushless motors equipped with a rotor of the permanent magnet type.
First, a device for controlling a brushless motor will be described.
To control brushless motors, instead of employing a position detection element such as a Hall element to detect the relative position of the stator winding and permanent magnet type rotor, a system has come to be employed wherein this position is detected by using the terminal voltages, which contain the induced voltages generated in the stator winding.
A prior art example of this is shown in FIG. 22. The numeral 1 denotes a DC current source. The numeral 2 denotes an inverter circuit for passing current to stator windings 3U, 3V and 3W of brushless motor 3. Numerals 4, 5 and 6 denote filter circuits that shift by 90 degrees the phase of terminal voltages UV, VV, and WV containing the induced voltages generated in stator windings 3U, 3V and 3W. Numeral 7 denotes a detection circuit whereby the neutral point voltage NV is obtained from the output signals of these filter circuits 4 to 6. Numerals 8, 9 and 10 are comparators that respectively compare the output signals of filter circuits 4 to 6, which constitute first order delay elements, and neutral point voltage NV. Numeral 11 denotes a control circuit.
FIG. 23 is a timing chart showing the operation of the prior art example. We shall now consider the U phase with reference to this timing chart. During commutation of the inverter circuit 2 the terminal voltage UV (see FIG. 23(a) generated in stator winding 3U contains a voltage spike that is produced by conduction of the opposite arm return flow diode. In order to eliminate the effect of this voltage spike component, the terminal voltage UV is shifted in phase by 90 degrees by means of filter circuit 4, producing the phase-shifted voltage DUV as shown in FIG. 23(b). After this, this phase-shifted voltage DUV and the neutral point voltage NV shown in FIG. 23(b) are compared by comparator 8, to obtain a phase detection signal PSU as shown in FIG. (23c). The situation is the same for the other phases V and W, position detection signals PSV and PSW being obtained as shown in FIG. 23(d) and (e) from comparators 9 and 10, based on terminal voltages VV and WV. These position detection signals PSU, PSV and PSW are signals that are 120 degrees different in phase for 180 degrees conduction. By applying these to control circuit 11, this control circuit 11 is made to output six commutation signals, which are applied to the bases of the transistors that constitute the switching elements of inverter circuit 2.
However, in the device described above for controlling a brushless motor, since filter circuits 4 to 6 having a 90 degrees lagging phase characteristic are provided in order to remove the voltage spike component contained in terminal voltages UV, VV and VW, the time constants of filter circuits 4 to 6 are large. This gives rise to the problem that it is not possible to track rapid acceleration or deceleration. A further problem in that phase detection is the low speed region is difficult. Furthermore, the size of the voltage spike component contained in terminal voltages UV, VV and WV varies depending on the magnitude of the current, i.e., the load of stator windings 3U, 3V and 3W, so if the load fluctuation is large, a phase error is produced in the signal waveform of filter circuits 4 to 6 et seq, causing a stability problem.
Next, a device for controlling a three-phase induction motor will be described.
This type of conventional device is disclosed Japanese Laid Open Patent Publication No. 62-100192, and is illustrated in FIG. 24.
A voltage type PWM inverter 12 consists of a rectifier circuit 14 that rectifies the three-phase AC voltage of three-phase AC power source 13, a smoothing capacitor 15 that smooths this rectified voltage, and a main inverter circuit 16 to which this smoothed DC voltage is applied. The AC output voltage from main inverter circuit 16 is then applied to three-phase induction motor 17. The current Idc flowing in the DC bus of inverter 12 is detected by current detector 18 and supplied as detection current I to low pass filter (LPF) 19. LPF 19 extracts the fundamental wave of the detected current I and outputs it as frequency correction value .DELTA.f. Subtractor 20 subtracts this frequency correction value .DELTA.f from the frequency command value f* to give a reference frequency value f (=f*-.DELTA.f), which is output and applied to a pulse width modulation (PWM) control circuit 21. This pulse width modulation control circuit 21 is supplied with a voltage command value V* obtained by converting the frequency command value f* by means of a frequency-voltage (f-V) conversion circuit 22. As a result, pulse width modulation control circuit 21 performs pulse width modulation control by applying a base signal to the power transistors of main inverter circuit 16, based on reference frequency value f and voltage command value V*.
Thus, since, in a voltage type PWM inverter, the DC voltage is fixed, the mean value of the current flowing in the DC bus of inverter 12 is proportional to the power supplied to three-phase induction motor 17. In this case, assuming that the rotational speed of three phase induction motor 17 is sufficiently high, and that the rate of change of the speed of rotation is very small, torque fluctuation is proportional to power fluctuation controlling the mean current of the DC bus of inverter 12, torque control of three-phase induction motor 17 can be performed, and production of vibration can be prevented.
However, although the above control device is effective when the speed of revolution of the three phase induction motor 17 is sufficiently high, and the ratio of variation of speed of revolution is very small, during low speed operation, when the speed of revolution is low, the amount of power change for a given change of torque falls off. Even if this is therefore compensated by dividing by the speed of revolution or by the inferred value of the speed of revolution so as to remove dependence on the speed of operation, the lowered S/N ratio means that sufficient accuracy is not obtained. Stable driving of the inverter during low speed operation is therefore difficult.